1. Field of the Invention
The field of the invention relates to a medical imaging method of tomosynthesis. In particular, it relates to a method for processing images obtained by tomosynthesis to improve the quality of the images necessary for a radiologist to develop a diagnosis.
2. Description of Related Art
Tomosynthesis is a medical imaging modality whereof the essential principles are listed hereinbelow and illustrated in FIG. 1.
FIG. 1 schematically illustrates the acquisition of 2D images of the organ and reconstruction of a 3D image of this organ by tomosynthesis.
X-rays, R, originating from a source S are emitted according to different angulations (1, . . . , i, . . . , n) to the organ O. After passing through the organ, they are detected by the detector Det forming a set of projection images (Al, . . . , Ai, . . . , An). It should be noted that there are as many 2D images acquired as angulations considered.
In single-energy tomosynthesis the basis of the 2D images is supposed to be locally uniform around the lesion of interest.
In the case of dual-energy tomosynthesis it is evident that two images are acquired with two different energy spectra for each angulation considered and that a contrast medium has been earlier injected in the patient.
The acquisition is executed by a detector Det situated opposite a source of X-rays, for example a digital camera or a solid detector based on amorphous silicon or amorphous selenium.
Application of tomosynthesis is detection and characterisation of a lesion in an organ, for example a cancerous lesion.
In a tomosynthesis embodiment the 2D projection images are utilised to reconstruct a 3D image, and the radiologist interprets this image as a function of the differences in contrast observed.
The reconstructed volume (VR) contains a plurality of voxels quantifying the attenuation of X-rays by matter encountered.
In the case of an organ exhibiting a lesion it presents attenuation different to the organ.
The result on the 3D image is a contrast difference for detecting the lesion.
However, an intrinsic problem to tomosynthesis is the limited angular opening of the acquisition of projection images which causes in particular limited resolution according to the dimension perpendicular to the plane of the detector. Here the plane of the detector is according to the x, y axes of an orthonormal marker and the perpendicular dimension is according to the z axis of this same marker.
Poor resolution in the z dimension perpendicular to the plane of the detector is due to the limited angular opening covered by the X-ray tube in comparison to tomography.
In fact, to obtain isotropic resolution (identical according to all dimensions) a set of projection images right around the organ would have to be acquired continuously. Yet it is understood that this is practically impossible for tomosynthesis examination.
Methods are known for improving the resolution of images acquired for discriminating nearby elements.
But apart from this problem, beyond a certain thickness of a lesion (taken along the z axis), the value of the voxels making up this lesion in an image plane passing through the lesion in the reconstructed volume is not sufficient for improved identification of the type of lesion by the radiologist (benign or malignant lesion).
The effect of this is to introduce uncertainty to the characterisation of the lesion, which can obviously compromise diagnosis.
In other terms the reconstructed image is reliable only for lesions having minimal thickness relative to their size according to a plane which is parallel to the detector and which passes through the object.